Dielectric heating with cavity resonator



Patented Apr. 18, 1950 UNITED STATES PATENT OFFICE DIELECTRIC HEATINGWITH CAVITY RESONATOR Application October 4, 1946, Serial No. 701,300

Claims.

Our invention broadly relates to dielectric heating apparatus; but moreparticularly relates to the application of cavity resonators tohighfrequency dielectric heating apparatus.

An object of our invention is to provide dielectric heating apparatusincluding a resonant cavity, more specifically, a reentrant resonantcavity.

Another object of our invention is to provide a dielectric heatingapparatus of a type described having a region with a high intensityelectric field into which the material to be heattreated can be readilyplaced and removed.

A more particular object of our invention is to provide a dielectricheating apparatus comprising a resonant reentrant cavity having an outershell at ground potential and a readily accessible work-receiving spaceinside the outer shell. The shell protects an operator against exposureto the high-voltage parts of the equipment.

In one form of our invention, part of the outer shell is movable forloading the work-receiving space of the apparatus. This part must berestored in order to place the apparatus in proper operating condition.In this embodiment, the resonant cavity is provided with an innercentral member having a face spaced from a parallel face of an end wallof the shell, the end wall being movable so as to permit the apparatusto be loaded and unloaded. This end wall is at a current nodal point sothat the joint between it and the rest of the shell does not carry anyconsiderable current, and a loose joint can be tolerated, although aclose fit is recommended.

In another embodiment of our invention, which forms the subject matterof our divisional case, Serial No. 107,442, filed July 26, 1949, theresonant reentrant cavity takes the form of an injecting apparatussuitable for pre-heating or otherwise treating plastic material or thelike, which is forced from the apparatus into a mold. In such anembodiment, a part of the resonant cavity is provided with an injectionnose or nozzle.

Features, innovations and objects of our invention, in addition to theforegoing, will be dis-,

cernible from the following description which is to be taken inconjunction with the accompanying schematic drawing in which:

Figure 1 is a central longitudinal view, mostly 2 an injection equipmentin accordance with our invention, and

Fig. 4 is a sectional view substantially on the lines IV-IV of Fig. 3.

In the embodiment shown in Fig. 1, a resonant cavity or chamber of thereentrant type is shown. This reentrant resonant cavity is indicated inits entirety by the reference numeral 2 and comprises an outer member orshell having a hollow longitudinal tubular wall 4 and opposite end walls6 and 8, respectively. The walls are preferably of metal so that theshell constitutes an outer metallic enclosure which can be readilygrounded so as to be at ground potential. The end wall 5 is unitary withan edge of the tubular wall 4, and has secured thereto, or is integralwith, a solid metallic central member H! which coaxially extends intothe tubular wall 4 and is symmetrically arranged with respect to it. Theother end wall 8 is snugly slidable inside the tubular wall 4 and can beforcibly moved to any desired distance from the central member ID so asto constitute a plunger. The separation between the inner face or worksurface I 2 of the end wall 8 and the facing transverse face or worksurface 14 of the central member ID provides a work-receiving space inwhich dielectric work W, comprising a plastic or any other suitabledielectric, may be placed for heat-treatment under a pressure of a valuedetermined by the force exerted on the outer side of the end wall 8.This force is schematically indicated at Fig. 1 by the series of arrowsand a plunger or rod 16. In order to resist this force, the other partsof the cavity would, of course, have to be braced.

When the cavity resonator 2 is made to resonate, a current anti-nodewill be at the end wall 6. A high voltage will exist between theseparated faces l2 and M at the other end of the cavity resonator, andthis high voltage will dielectrically heat any work between the faces.

The portions of the end wall 8 and the central member ID between whichthe work is held or pressed can be considered to be relatively insulatedheating electrodes, such as are found in more conventional dielectricheating apparatus. The electrode or end wall 8 can he slid into and outof the tubular wall 4 so that the dielectric heating equipment can beloaded and unloaded. However, it is obvious that the upper portion ofthe equipment could be moved instead, or even the central member onlyslid through a hole in the end wall 6. This last form, however, has thedisadvantage that the sliding joint will be at a current anti-nodalpoint so that unless an extremely good conductive connection existsacross the joint, power would be lost unnecessarily.

The shell can be made of any suitable material but preferably it. shouldbe designed so that. it will absorb littlegpower itself; To thisend, thecavity may be of'steel and provided with coatings of copper or silver orother metal of high conductivity and low permeability. If desired,however, the entire resonant cantycanbenaoe of copper.

The cavity resonator can be po-Weredby making it part of the tankcircuit of an oscillator trey coupling a conductor loop for acapacitorplate to it in a customary manner. The upperlimito'f the frequency atwhich the cavityresonator operates is governed by the size of the workto be -heat treated between faces I2 and M. For a given set ofconditions, the less the capacity between ithe faces, the higher will bethe frequency at which with? resonator will oscillate. v 7

The apparatus has an importantadvantage in .that it-providesa-highdegree of safety for operation because the outer shellconstitutes: an engclosurawhich can easil be kept at-groundpotenr tion.T-Asafur'ther precaution, any suitable-safety devic would; be I attachedto the movable wall; 8 nor interrupting the power to the cavityresonator i'wl ienever the wall-starts to move from the heatfirra nosiuqn- ,T-he cross-section of the reentrantcavity of'the ,resqnator can:beselected as desired. -T-hemost ;convenientjform is to make both thecentral memllland the tubular member t circular and con- -cent1ic, Insuchase, the endwalls -6. and 8 would necessarily be round discs.However, we

{hate usedacavity resonator in which'the central gmern -ber and outertubular member; were square incross-section, and had their-facing sidesa li l i a. v n

l i an indication of the operation of our invention, wen-have heated in;a -matter of seconds, *a "plastimpreform 9 india meter, 2V thiclc andihavingf a loss factor of ..6 to Lain-areentrant resonantcavity, of atype shown in Fig. ':1,; which was square inv eross-s eetiorr and hadthe following -dinien sionsj inside overall length -30", central inmb'er =12 squareson 'the outside, outer tubular ber :18 sduareon theinside, I This reentrant I eso'nantc'avity operated at thefrequencies-from ith the, electrode surfaces or faces l-2 and l4 that,FtheIVoltag eP between them .will be greatestrat l'the enter andbecomesprogressively smaller be- "their circumference or periphery. Withifiadfcircular electrode surfaces, the V'Oltagebetween any two.longitudinally aligned points on the :lectfodesurfaces l k-andlllwill-depend on the ;radial' distance .from'the axis or center. -A.good

iapprbximauoa ofithe voltage distribution canbe obtained from the.following equation:

.rWhQI'e Emax'lS the-voltage-across the two center pointsofthe'surfaces'e'i's the dielectric constant *of-thework; x is-the wavelength of "theoscili'a- --tions,an-d is the voltage 'at the distance; r,

from the center.

i The amount of heating per unit volumeofwork 3 variesas the square ofthe voltage :so ithat the radial voltage variation I may in some casescause non-uniform heating. =-Ifr the variation large enough to v be;objectionable, 'amore uniform heat- .ing can. be obtained --by i dishingoutone iofwthe electrode surfaces as indicated at 20 in the emthe facingheating surfaces will be partially ab- "s'orbed in the filling 22, andthe remainder in modified from the embodiment of Fig. 1.

the wornw.

'Fig. 2 shows other features which have been Thus, theiihsulatedjend orthe central member ID is fprovided with an-annular outwardly projectingpor-tionor flange '24 so as to be able to heat work :of larger surfaceordiameter. Also, the end wall '8 is .hinged, bya hinge 26, to the outertubular wall! and can be'forcibly latched closed by a latch28.

Teachings of our invention are also applicable to injection; molding,andin -Fig. 3ap aratus of this typeis schematically indicated in itsentirety by :thereference numeral 313. :It-comprisesa metallic reentrantcavity havinganouter tubular wall 3-2, an annularfend wall. 3d and anopposite annular transverse closing endwalle fi. Extending from theinner edge (if-the endv wall '3 is a hollow central tubular member 38. Ametallic streamlinedelectrode member 40 is-connected to the far-orinsulated end offthe central tubular member 38 by spaced. metalconnecting bars 42.

=The end wallet is provided with anoutwardly directed. metallic nozzleid insulated from and spaced from the associated end of the electrodemember 38 by a-stepped ringffi of insulation. The inside .of the "nose Mis provided. with a suit- ;ably. shaped face 'or. surfaceliifwhichcooperates with a-soniewha't s'iinilarlylshape'dface or surface 50. onthe member 40. to form. a work-receiving spaceacrosslwhich.a'high-frequency voltage exists. The 'insidebf the. centralmember 3% is adapted to receive pellets or other suitable :plastic'maten rs'z .which'can beiorced by;a--ram,-or .plunger'54in'tothework-receiving space between 'lthe faces 48 .andbllbffthenoseldandmember 42,

. chamber having. outer. wallswincluding; 'an end -.-wa lland having-aninnerz-reentrantiportion facing said end 'wall but spaced therefrom toprovide :a work-receiving: space; said end wall: being Zmdvable withrespectto said-reentr'ant portion and to other walls of saidch'ambefsosas to provide wall and having an inner reentrant portionfacing said end wall but spaced therefrom to provide a work-receivingspace, said end wall being movable with respect to said reentrantportion and to other walls of said chamber so as to provide an openingin said chamber for loading work into said space for heat-treatment andunloading said work after such heat-treatment, and a pressure-exertingmeans engaging the outside of said end wall for holding work betweensaid end wall and said reentrant portion, said pressure-exerting meanscomprising a member connected to the outside of said end wall andapplying pressure on the outside of said end wall.

3. Dielectric heating means comprising means providing a resonantchamber having means through which it may be electrically excited, saidchamber having outer walls including an end wall and having an innerreentrant portion facing said end wall but spaced therefrom to provide awork-receiving space, said end wall being movable with respect to saidreentrant portion and to other walls of said chamber so as to provide anopening in said chamber for loading work into said space forheat-treatment and unloading said work after such heat-treatment, and apressure-exerting means engaging the outside of said end wall forholding work between said end wall and said reentrant portion, saidouter walls comprising a tubular wall, and said end Wall slidablyfitting inside of said tubular wall.

1. Dielectric heating means comprising means providing a resonantchamber having means through which it may be electrically excited, saidchamber having outer walls including an end wall-portion and having aninner reentrant portion including an end wall-portion facing the firstsaid end wall-portion but spaced therefrom to provide a work-receivingspace, one of said end wall-portions being relatively movable withrespect to the other and to other walls of said chamber so as to providean opening in said chamber for loading work into said space forheat-treatment and unloading said work after such heat-treatment, and apressure-exerting means engaging the outside of said movable endwall-portion for holding work pressed between said end wall-portionsduring heat-treatment of the work, said pressure-exerting meanscomprising a member connected to the outside surface of said movable endwall-portion applying pressure to the outside of said movable endwallportion.

5. Dielectric heating means comprising means providing a resonantchamber having means through which it may be electrically excited, saidchamber having outer walls including an end wall-portion and having aninner reentrant portion including an end wall-portion facing the firstsaid end wall-portion but spaced therefrom to provide a work-receivingspace, one of said end wall-portions being relatively movable withrespect to the other and to other walls of said chamber so as to providean opening in said chamber for loading work into said space forheat-treatment and unloading said work after such heat-treatment, wallsof said resonant chamber providing a tubular portion in which one ofsaid end wall-portions is slidable, and a pressure-exerting memberassociated with the outside of said slidable end wall-portion applyingpressure thereto during heat-treatment of the work in said space.

THOMAS W. DAKIN. CARROLL N. WORKS. FITZHUGH BOGGS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS FOREIGN PATENTS Country Date Great Britain Sept.5, 1946 Number Number

